Skip-spread method for seismic surveying

ABSTRACT

A method of seismic data acquisition wherein the average speed of data acquisition is rapid and the time and total cost necessary for data acquisition are maintained at relatively low levels. A seismic cable may be towed during an activated portion of its travel at a speed sufficiently slow to achieve optimum results of data acquisition and may be towed at a much greater speed during a deactivated portion of its travel thereby causing its average towing speed to be faster than is ordinarily practicable thereby reducing costs of data acquisition without adversely affecting the quality of the seismic data acquired.

ilnited States Patent [191 Mayne et al.

[ Felo.5,i974

[ SKIP-SPREAD METHOD FOR SEISWC SURVEYING [21] Appl. No.: 190,478

[52] US. CL... 340/l5.5 MC, 340/7 R, 340/l5.5 CP [51] int. Cl. Glllv1/24 [58] Field of Search. 340/15.5 CP:l5.5 MC, 7

[56] References Cited UNITED STATES PATENTS 4/1969 Proffitt 340/l5.5 MC10/1963 Jolly IMO/15.5 MC 11/1967 Cetrone et al.... 340/l5.5 MC

V x 0 0 o o o o o o o O 0 O 0 o o O o o O O o O O V X x o o o o 0 c o o0 o 0 O 0 o o 0 0 o 0 0 o 0 0 V X X l 0 o 0 0 0 o o o O O O o 0 o o O cO 0 o o O O O TOW 76 I 12 14 24 36 f i 2, E

SLOW CONSTANT 3,412,373 11/1968 Ellis ..340/l5.5 MC

Primary Examiner-Samuel Feinberg Assistant Examiner-H. A. BirmielAttorney, Agent, or Firm-Tom Arnold et al.

[57] ABSTRACT A method of seismic data acquisition wherein the averagespeed of data acquisition is rapid and the time and total cost necessaryfor data acquisition are maintained at relatively low levels. A seismiccable may be towed during an activated portion of its travel at a speedsufficiently slow to achieve optimum results of data acquisition and maybe towed at a much greater speed during a deactivated portion of itstravel thereby causing its average towing speed to be faster than isordinarily practicable thereby reducing costs of data acquisitionwithout adversely affecting the quality of the seismic data acquired.

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VXIXXX(XXOOQOOOOOOOOOOOOOOOOOOOOO (I! VIIXX(XX(000000000OOOOOQOOOOOOOOOXit v xxixi il ooooooooooaoooooooooooo X7(xi,\Xxix/(x100000000000000000000000 INCREASED SPEEDxxrxxxxixiixiXrtrxxiixxxxrvXKYXXYYYXY rxxrixxxiitiiixxxilyxixixxxiliitxxxi l l V ooooocooooaoooooooococoo V000000000000000000000000\XXtilllXXX VXA cocoocooooocooooocoocoo VXooooocoooocooooooo000000 vXxxlxX7ooooooooooooor' xXXXXXvxxXi V SOURCELOCA TION q DETECTOR LOCATION x UNOCCUP/ED OR NON "ACTIVE LOCATION rixiACTIVE DETECTORS o o o o o o o O o o o o o c o o o o o o o o o y x i V YX x X x i x l I X 0 o o o o o o o o 0 o O 0 o o o 0 o n o o o 0 03g 2 Xx x x r x x x x x x? o o o o o o a a o o o o o o o o o o o o o o c KDETECTORS PATENIEB FEB 5 i974 SHEH USO? 8G UUUUUUUUBUU MN NN NW QN Q o m0 Mm mm a QN ooo 050 Nmwumo 0 c o o o o o a Q o o Nmwmao BUD O O D O O oo o o o O N OYILO O wU U U U U m mik .2501 ZOEUMIMF WQ 0 553a SIDE UUUDU SKlP-SPREAD METHOD FOR SEISMIC SURVEYING BACKGROUND OF THEINVENTION This invention relates to seismic data acquisition and moreparticularly to marine seismic data acquisition that is accomplished insuch manner as to achieve horizontal stacking data acquisitiontechniques at rapid speed and low cost. The invention relates to bothmarine and land surface techniques, but for purposes of simplicity willbe discussed primarily as it relates to seismic exploration in a marineenvironment.

In marine seismic data acquisition, it is customary to employ a seismiccable having a plurality of detectors, which may also be referred to ashydrophones, located along the length thereof which are capable ofdetecting seismic waves being transmitted through earth formation andthrough a body of water. The seismic cable with its detectors is towedthrough the water by a vessel and is frequently referred to in theindustry as a seismic spread.

In operation, using the reflection method, a seismic wave is generatedin the vicinity of the seismic spread, which seismic waves or shockwaves, as they are frequently referred to, travel through the water tothe bottom of the body of water and are reflected back to the hydrophonedetectors which sense the reflected waves and cause recording apparatusto record a number of points corresponding to the number of activerecording channels connected at any one time to various ones of thehydrophones. The seismic waves may be generated by detonating anexplosive substance such as dynamite in the water or in the earth or byimpacting the earth with a weight device. Seismic waves may also begenerated accoustically by any of a number of commercially acceptablemeans. The length of the subsurface profile reflected is generally equalto one-half of the overall spacing of the active hydrophones in thehydrophones spread in view of the geometric pattern of the reflectedwaves that are detected by the hydrophones. Geological formations belowthe floor or bed of the body of water may also be detected throughreflections at earth surfaces where the density or other properties ofthe strata undergo abrupt changes.

It has generally been the practice to generate a seismic wave or shotpoint at a location offset from the line of hydrophones and at aposition intermediate the extremities of the seismic spread, althoughthe specific relation of a shot point to the seismic spread may bevaried in accordance with the type of seismic data to be acquired. Wherea seismic cable is towed, it may be appropriate to generate the shotpoint at a specific point relative to the spread and therefore theseismic wave generation device may seismic cable.

The vessel towing the seismic spread may stop briefly while the seismicwave generation and recording is accomplished or it may be practical tocontinue movement of the seismic spread at a relatively slow speed andcompensate mathematically for discrepancies in the data acquired thatmight result due to such move ment.

Where the seismic spread is continually towed through a body of waterduring generation of seismic waves and recording, a certain amount ofnoise is created by movement of the seismic spread and this noise bephysicially connected to theis detected by the hydrophones and relayedto the recording equipment where it may interfere with seismic databeing reflected from the ocean bottom and from various subsurfacestrata. Consequently, it is necessary to maintain movement of theseismic spread through the water at a constant and relatively low speedat which an acceptably low noise level is produced. It is obvious thatthe average speed of a seismic traverse conducted at a slow constantspeed will be correspondingly limited and the cost of seismic dataacquisition will be relatively high.

During seismic exploration, it has been found desirable to employ a dataacquisition technique generally referred to as the common reflectionpoint stacking method to eliminate the false seismic indicationsoccasionally produced by multiple reflections that may occur when thereexists a sharp discontinuity both at the bottom of the ocean and at adominant strata below the bottom of the ocean. Under these adverseconditions seismic waves pass through the bottom of the ocean and arereflected from the dominant strata back to the bottom or surface of theocean and then the seismic waves are reflected by the bottom or surfacedownwardly again to the dominant strata and they are finally reflectedupwardly through the ocean floor to the hydrophone array. Multiplereflections of this nature give a false indication of an additionaldiscontinuity located below the dominant strata by a distance equal tothe spacing between the ocean floor and the dominant strata. Suchmultiple reflections also frequently override and obscure primaryreflections from deep strata. The common reflection point stackingtechnique is also employed to reduce the effects of towing noise as theseismic spread is towed through the water. The noise created by rapidmovement of a hydrophone spread through a body of water, of course, alsocomplicates the common reflection point stacking technique and generallyrequires the towing speed to be quite low.

It is, accordingly, a primary object of the present invention to providea novel method of seismic data acquisition involving the towing of aseismic spread that allows overall increase in towing speed of a seismicspread without increasing noise level above an optimum value.

It is an even further object of the present invention to provide a novelmethod of marine seismic data acquisition that allows data acquisitionto be accomplished at constant acceptably slow speed, producing minimumtowing noise, therebyproducing optimum data acquisition results.

Among the several objects of the present invention, is noted thecontemplation of a novel method of marine seismic data acquisition thatis accomplished by employing the horizontal stacking technique and yetmaintains average speed of towing a seismic spread at a high ratewithout creating excessive noise during periods of data acquisition. t

It is another object of the present invention to provide a novel methodof marine seismic data acquisition that achieves optimum horizontalnoise cancellation due to close spacing of the traces.

It is an even further object of the present invention to provide a novelmethod of marine seismic data acquisition that allows employment of thecommon reflection point stacking technique and high speedof tra versewithout increasing spacing between common reflection point files.

It is an even further object of the present invention to provide a novelmethod of marine seismic data acquisition that is capable of achievingefficient velocity analysis.

Another object of the present invention contemplates the provision of anovel method of marine seismic data acquisition that achieves multiplecoverage without sacrificing speed of the seismic traverse or quality ofthe seismic data acquired.

It is an even further object of the present invention to provide a novelmethod of marine seismic data acquisition wherein seismic data isacquired at a rapid rate thereby reducing resultant cost of seismic dataacquired.

Other and further objects, advantages and features of the presentinvention will become apparent to one skilled in the art uponconsideration of the written specification, the attached claims and theannexed drawings. The form of the invention, which will now be describedin detail, illustrates the general principles of the invention, but itis to be understood that this detailed description is not to be taken aslimiting the scope of the present invention.

THE PRIOR ART Since the granting of U.S. Pat. No. 2,732,906 in 1956,many patents have been granted on various methods of recording andcombining seismic data to achieve multiple fold stacked data. Forexample, the recorded traces may be weighted or mixed in order toachieve a more continuous sequence of events as taught by Ehlert et al.in Pat. No. 3,181,643 or different dipped angles of the reflecting bedsmay be determined and utilized to identify the reflections prior to thecombining of seismic data into a stacked section as set forth in thepatent to Mendenhall et al. No. 3,217,828. Alternatively, as taught bythe patent to Strange, No. 3,133,262, multiple spreads may be employedhaving differing detector intervals at different water depths to recorddeep or shallow reflections.

SUMMARY OF THE INVENTION A preferred method of seismic data acquisitionaccording to the present invention may be accomplished either on land orin a marine environment although the invention is particularly directedto use of the method in a marine environment for purposes of simplicity.The method may include the provision of a seismic spread having agreater number of detector stations thereon than the number of activerecording channels to be employed for data acquisition during any givenrecording. The active recording channels are connected to selected onesof the detector stations and after each seismic wave generation andrecording sequence, the active recording channels are mechanically orelectronically switched to different detector stations in a prescribedmanner upon successive shots to generate seismic waves. The activerecording channels are switched to different detector stations onsuccessive shots in such manner as to obtain a desired fold ofmultiplicity on a stationary subsurface coverage from a series ofclosely spaced records taken as the seismic spread and source of seismicwaves generation are towed at a constant but acceptably slow speed. Inuse of the method of this invention, desired multiplicity will beobtained over a subsurface distance equal to one-half of the length ofthe seismic spread and therefore, it will be possible to skip therecording of data over a distance equal to one-half of the length of theseismic spread. Since no recordings are being made during the skippedphase, the rate of tow can be increased significantly over thenonrecorded distance with a corresponding increase in overall averagespeed without adverse affect on the records that otherwise might occurdue to excessive noise caused by towing the seismic spread through abody of water.

Alternative forms of this invention do not require a greater number ofdetector stations than the number of active recording channels andtherefore would not necessarily employ a switching means.

BRIEF DESCRIPTION OF THE DRAWINGS So that the manner in which the aboverecited features, advantages and objects of the present invention, aswell as others which will become apparent, are attained and can beunderstood in detail, more particular description of the invention,briefly summarized above, may be had by reference to the preferredembodiments thereof illustrated in the appended drawings, which drawingsform part of this specification.

It is to be noted, however, that the appended drawings illustrate onlytypical embodiments of the invention and are therefore not to beconsidered limiting of its scope for the invention may admit to otherequally effective embodiments.

IN THE DRAWINGS FIG. 1 is a graphic presentation of a seismic traverseaccording to the present invention involving 36 detector stations andillustrating movement of the source point, switching of the activerecording channels, and inactive movement of the seismic spread duringthe traverse period.

FIG. 2 is a graphic illustration of source locations during a seismictraverse conducted in accordance with the present invention.

FIG. 3 is a graphic representation of reflection point data, shown to bel2-fold coverage, obtained by employing the method illustrated in FIGS.1 and 2.

FIG. 4 is a graphic representation of an alternate method of seismictraverse according to the present invention involving 24 detectorstations and illustrating movement of the source point, switching of theactive recording channels and inactive movement of the seismic spreadduring a seismic traverse.

FIG. 5 is a graphic illustration of source locations during a seismictraverse conducted in accordance with FIG. 4.

FIG. 6 is a graphic representation of reflection point data, shown to bel8-fold coverage, obtained by employing the seismic data acquisitionmethod illustrated in FIGS. 4 and 5.

FIG. 7 is a graphic representation of a skip spread seismic traverseconducted in accordance with the present invention, involving 24detector stations and illustrating movement of the source point,switching of the active recording channels and inactive movement of theseismic spread during a shoot-six-skipl 2 seismic traverse.

FIG. 8 is a graphic representation of source locations during theseismic traverse illustrated in FIG. 7.

FIG. 9 is a graphic illustration of reflection point data obtained byemploying the seismic data acquisition method illustrated in FIGS. 7 and8 and shown to be six-fold multiple coverage.

FIG. 10 is a graphical representation of an alternate mode of operationfor use when the switching mechanism is not employed and the number ofthe detector stations is equal to the number of recording channels.

FIG. 11 illustrates the operation of the procedure in FIG. 10 andincludes the subsurface reflecting points.

DESCRIPTION OF PREFERRED EMBODIMENTS Now referring to the drawings andfirst to FIG. 1, there is illustrated a seismic spread 10 comprising aseismic cable 12 having 36 seismic wave detectors situated in evenlyspaced relation along the length thereof. For purposes of the presentinvention, any one of a number of commercially available seismic wavedetectors may be employed within the scope of the present invention.Such detectors, when employed on land, are generally referred to asgeophones or phones and when employed in a marine environment, aregenerally referred to as hydrophones.

A source 16 for the generation of seismic waves, also referred to as ashock wave, may be connected to seismic cable 12 ahead of the first oneof the seismic detector stations 14. The source 16 may be any one of anumber of acceptable means for generating seismic waves, such as anexplosive device, an acoustical wave generating device or the like,within the scope of the present invention.

The seismic spread 10 may be towed through a marine environment by avessel or may be propelled or otherwise manipulated in a land basedenvironment to achieve movement in the direction of the arrow in FIG. 1.For purposes of simplicity, to facilitate understanding of theinvention, discussion will be related particularly to application of theinvention in a marine environment, although it is not intended that theinvention be restricted to use in marine environment.

The distance between the source 16 and the first detector station isshown to be equal to the detector interval among the 36 detectorlocations but this particular source-to-first-detector-distance is notessential to the ends of the present invention. The distance between thesource location and the first detector is not restricted to any givenlength (but some distance lengths will enhance the quality of a datarecording). The source to first detector distance however, must remainuniform throughout any given traverse. The detectors may be spacedevenly at 200 foot intervals along the length of the seismic cable, butagain this particular distance is not intended to limit the presentinvention, it being obvious that other detector intervals may beemployed within the spirit and scope of the invention.

To produce multiple fold coverage or horizontal stacking, as such isfrequently referred to in the industry, as the seismic cable is beingtowed through the body of Water at a constant slow rate, it is desirableto generate seismic waves and detect reflected waves in such manner thatmultiple coverage of the subsurface is obtained. As explained in anarticle published in Vol. XXVII, No. 6, Part llof Geophysics (Dec. 1962)by W. Harry Mayne, and titled Common Reflection Point Horizontal DataStacking Techniques, the commonreflection point technique was devised toprovide a practical means of increasing multiplicity to attenuate noisewithout increasing the subsurface area and obscuring the detail that issought by the survey. By employing the multiple coverage, commonreflection point technique, reflected signals, received along theseveral paths from the reflection point to the various wave detectiondevices, will produce a resultant sum that will be proportional to thenumber of signals received. Perturbations following other than thepostulated ray paths will not be coincident, and hence will be degradedfrom the reflections.

According to the present invention, a method of obtaining such multiplefold coverage may conveniently take the form illustrated in FIG. I wherea seismic spread is depicted in the uppermost line of FIG. 1 at theinitial point of a seismic data gathering traverse with detectorstations 1 through 24 activated while detectors 25 through 36 aredeactivated and do not transmit reflected data to a data controlfacility. To facilitate activation and deactivation of the detectorstations, it may be appropriate to provide a 24 channel recordingcircuit that may be selectively connected to the various detectorstations by a stepping switch or by any other conventional switchingapparatus of acceptable nature. The switching apparatus will beoperative to switch sequentially after each seismic wave to successivecontiguous groups of the detector stations as the seismic cable is towedthrough a body of water. In the alternative, it may be appropriate tocontinuously record from all of the 36 detector stations andsubsequently electronically select information from successivecontiguous groups of the detector stations in order to produce desiredmultiple fold coverage of the seismic data.

In line 2 of FIG. 1, the source location or source point 16 is depictedas having moved forwardly or to theleft a distance equal to one-half ofthe detector interval while the switching apparatus has deactivated thefirst detector station and has activated detectors 2 through 25 and hasallowed detectors 26 through 36 to remain in a deactivated state. It isapparent that while the physical movement of the seismic cable 12 in aforward direction is equal to one-half of the detector interval, thecontiguous group of active detectors has moved rearward or to the rightby an effective distance equal to one-half of the detector interval. Theeffective subsurface area covered for data reflection is therefore equalto one-half of the distance of cable movement. The seismic spreadcontinues in its travel a sufficient distance to cause 12 sourcelocations or source points at which time detector stations 1 through 11will be deactivated while stations 12 through 35 will be electricallyconnected to the 24 channel recording circuitry.

With reference now to FIG. 2, the activated portion of the seismictraverse is indicated by small triangular symbols as the seismic spreadis towed in the direction of the arrow at the left portion of FIG. 2.Vertical lines 19, 20, 21 and 22 separate the active source locationsfrom the inactive or skipped source locations as the tow physicallyprogresses along the path established by profile markers 24. v

The resultant reflection point data acquired by the first 12 activesource locations in FIG. 1 are depicted at the upper right portion ofFIG. 3, illustrating that 12- fold coverage, which may be referred to ashorizontal 12-fold horizontal stacking, is achieved by the presentmethod of seismic data acquisition.

It is important to note that the source 16 and the seismic cable 12continue to be towed during the active phase of seismic data acquisition(the 12 activated source locations) at a constant speed that issufficiently slow to prevent the development of excessive noise thatmight otherwise interfere with effective data acquisition. By deletingtraces through the mechanical circuit switching or electronic selectionmethods and by progressing the source and detector cable, a 12-foldstack is completed after 12 source locations have been activated. Sincea 12-fold stack has been completed, it is not necessary to continue dataacquisition through the succeeding 12 source locations. Accordingly, theentire seismic spread may be deactivated and the cable may be moved adistance equal to 12 source locations and seismic data acquisition mayagain begin with activation of detectors 1 through 24 as discussedabove.

Since the seismic spread may be moved a distance equal to 12 stations ina deactivated condition, in order to increase the average speed ofseismic data acquisition, it is appropriate to accelerate the speed ofthe vessel to the greatest practical extent during the skipped sourceportion of the traverse. Upon reaching the position at which seismicdata acquisition may again begin, the speed of the vessel is slowed toan optimum speed for data acquisition. It is noteworthy that the sourceand the detector cable continue to progress the traverse at a constantmoderate speed as the switching arrangement takes place and that after12 activated source locations and 12 dead or nonshot locations aretraversed at increased towing speed, the process is repeated. Thus, thesource is activated 12 times as the seismic spread is moved at aconstant slow speed and then 12 source locations are skipped duringwhich period the speed of the traverse can be increased substantially,thereby effectively increasing the average speed of the total traverseand reducing the average cost of data acquired. Even though the speed ofthe traverse is alternately increased and slowed, stacking of thedesired multiplicity is effectively obtained.

Continuous multiple fold coverage is effectively obtained primarilybecause the source and seismic cable continue to progress in onedirection while the activated detector locations effectively move in theopposite direction at the same effective speed thereby allowingreflections from common reference points to be effectively received bythe recording channels.

With reference now to FIGS. 4 through 6, there is depicted a modifiedembodiment of the skip-spread seismic traverse method of dataacquisition employing 24 detector stations and achieving l8-foldmultiple coverage.

A seismic spread is illustrated at 26 in FIG. 4 including a seismiccable 28 having 24 detector stations 30 connected in evenly spacedrelation along the length of the cable 28. The source 32 is illustratedby small triangles to indicate graphically the position at which seismicwaves are generated during traverse of the seismic cable 28 in thedirection of the arrow depicted at the upper left portion of FIG. 4. Theseismic data acquisition method illustrated in FIG. 4 may be referred toas the shoot-six-skip-six method, wherein a total of six active sourcelocations are generated while the seismic cable is towed at slowconstant speed by a vessel or the like. During the active sourcelocations, a switching circuit or an electronic data selection mechanismis employed to switch a total of 18 contiguous recording channels inprogressive order between each source location. As illustrated in theuppermost line of FIG. 4, the small circles indicate that detectors 1through 18 are activated by a switching mechanism or the like whiledetectors 19 through 24 are deactivated. As illustrated in the secondline of FIG. 4 at the second source location in the first activesequence, detector No. 1 and detectors 20 through 24 are deactivatedwhile the switching mechanism electrically connects the detectors 2through 19 to the l8-channel recording circuit. Switching and continuousmovement of the seismic cable continues through the sixth sourcelocation, as illustrated at the sixth line in FIG. 4, where detectors 1through 5 and 24 are shown to be deactivated while detectors 6 through23 are electrically connected to the recording system. After this hasbeen accomplished, the result will be a symmetrical six-fold stack asillustrated at the upper right portion of FIG. 6.

The seismic cable and source is then moved at an increased speed througha distance equal to six source locations and the seismic wave generationand recording process is again initiated with detectors 1 through 18electrically connected to the recording circuit while detectors 19through 24 are reactivated. During the active phase of data acquisition,the seismic cable 28 will, of course, be towed at a constant speed thatis sufficiently slow to prevent excessive noise. Active source locationswill continue through the next six source locations or locations 13through 18 thereby generating seismic waves and acquiring seismic datashown in FIG. 6 as the centermost six-fold stack of seismic data.

After the second active phase of seismic data acquisition the seismiccable 28 may again be towed at rapid speed through a distance equal tosix source locations at which time the speed of the traverse must againbe slowed to a speed sufficiently slow for effective recording. Thethird active phase is again initiated at line 25 of FIG. 4 in similarmanner as discussed above, thereby acquiring seismic data illustrated atthe lower most sixfold stack of seismic data in FIG. 6.

As illustrated in FIG. 5, the traverse is accomplished by alternativeactive and inactive six source locations as the seismic cable is towedby the vessel. However, it should be noted that the speed of cablemovement will be relatively slow during the active phases, shown by thesmall triangles, while the speed of the cable may be increasedsubstantially during the inactive phases, indicated by small Xs. Theactive and inactive phases are separated graphically by vertical lines32 through 40 in FIG. 5.

With respect to FIG. 6, it is important to note that the upper, middleand lowermost six fold stack of reflection point data overlaps to define18-fold coverage. It is also important to note, however, that the18-fold coverage obtained is substantially higher in cost of gatheringseismic data per mile of traverse, because the distance of activatedconsecutive source positions is half that illustrated in FIGS. 1 through3.

With reference now to FIGS. 7 through 9, it is apparent that the methodof seismic data acquisition, according to the present invention, may beemployed to achieve a lesser fold coverage, and according to the presentinvention such lesser seismic coverage may conveniently take the formillustrated in FIGS. 7, 8, and 9 which depict a shoot-six-skip-twelvemethod of seismic data acquisition.

A seismic cable 34 is illustrated in FIG. 7 as being towed in thedirection of the arrow by a vessel or by any other suitable towingdevice. The seismic cable 34 is provided with 24 detector stations 36connected in evenly spaced relation along the length of the seismiccable. A seismic wave source 38 is depicted as being connected to theleading extremity of the seismic cable at a distance ahead of theleading detector equal to the average distance between the variousdetectors. The source 38, of course, may be disposed in any othersuitable distance from the leading detector as might be appropriate toenhance the quality of the data recorded. It is necessary, as indicatedabove, that the source to first detector distance remain uniformthroughout any given traverse.

As illustrated in the uppermost line of FIG. 7, a switching mechanism orelectronic data selection system will be connected to receive seismicdata transmitted from the first 18 of the detectors or hydrophones ofcable 34. Where a switching arrangement is employed, an l8-channelrecording circuit may be electrically connected to the first 18 of thedetectors in order to receive seismic data transmitted therefrom and totransmit the seismic data to an appropriate recording facility forvisual or electronic readout.

As illustrated in the second line of FIG. 7, the seismic cable 34 hasbeen moved forwardly, or to the left a distance equal to one-half of theaverage distance between the detectors and the source 38 has beenenergized to generate a seismic wave. Simultaneously, the switchingmechanism has operated to deenergize the first detector and detectors 20through 24 while detectors 2 through 19 have been energized for seismicdata acquisition. Seismic cable 34 continues to be towed at a speed thatis constant and is sufficiently slow to prevent the development ofundesirable noise. Movement of the seismic cable 34 and switching of therecording circuit will continue through line 6 where the detectors 1through 5 and 24 will be deenergized while detectors 6 through 23 willbe electrically connected to the 18- channel recording circuit. It willbe observed that, after the sixth source location, as illustrated inlines 1 through 6, there will be provided six-fold multiple coverage ofseismic data being received from common reflection points.

After the sixth source location, all 24 of the detectors will bedeenergized as towing of the seismic cable continues through a distanceequal to 12 additional source locations where the cable will bepositioned as illustrated in line 18. During deenergized towing of theseismic cable 34, the speed of the tow may be increased substantially,thereby effectively reducing thetotal towing time necessary to move theseismic cable through a distance equal to a total of 18 sourcelocations. As the seismic cable reaches the position illustrated in line19 of FIG. 7, the l8-channel recording circuit will again beelectrically connected to detectors 1 through 18 and the source 38 willgenerate a seismic wave that will be reflected by earth substrata to thefirst l8 detectors for recording. Seismic wave generation at the nextsix source locations, depicted in lines 19 through 24, will then occurwith switching of the l8-channel recording circuit to the variouscontiguous detector groups as indicated above. After the last sourcelocation, as indicated in line 24, the entire seismic spread will bedeenergized as towing continues at an increased speed through a distanceequal to l2 source locations to a position illustrated in line 36 whereseismic generation and switching of the detectors will again occur astowing is again slowed to a constant optimum speed as indicated above.

llll) With reference now to FIG. 8, which graphically identifies ashoot-sixskip-12 method of seismic data acquisition, towing of thedetector cable will progress as indicated by the tow arrow and seismicwaves will be generated as indicated by source location triangles alonga traverse defined by profile markers. Since the towing of the seismiccable may be conducted at an increased speed during a distance equal tol2 skipped locations and slowed for seismic data acquisitions during sixactive source locations, it is obvious that the average speed of the towwill be substantially greater than seismic data gathering at aconventional slow towing rate.

In view of the foregoing, it is apparent that the present inventionprovides a novel method of seismic data acquisition that effectivelyallows an overall increase in speed of towing a seismic spread withoutincreasing noise level above an optimum value.

Even though the average speed of towing the seismic cable issubstantially increased by employing the skip spread method of thepresent invention, the seismic data is gathered at a constant andacceptably slow speed thereby enhancing the value of data acquired. Eventhough the seismic cable might be towed at various speeds, it ispractical to achieve the horizontal stacking technique of seismic dataacquisition and to do so without increasing spacing between commonreference point files. Moreover, the invention achieves optimumhorizontal noise cancellation due to close spacing of the traces. Ingeneral, the invention effectively provides for seismic data acquisitionof superior value which may be obtained at much faster rates andtherefore at lower costs than has heretofore been practical. Theinvention therefore, is seen to attain all of the objects and advantageshereinabove set forth together with other advantages which will becomeobvious and inherent from a description of the instant seismic dataacquisition method together with apparatus that may be employed incarrying out the method. It will be understood that certain combinationsand subcombinations are of utility and may be employed without referenceto other features and subcombinations. This is contemplated by and iswithin the scope of the present invention.

This invention is particularly applicable to a technique foridentification of moveout errors. FIG. 9 illustrates the commonreflection points for the methods in FIG. 7 and FIG. 8. The commonreflection point 101 consists of traces having six differentsource-todetector distances equal to one detector interval, two detectorintervals, three detector intervals, etc., to six detector intervals.This common reflection point 101 provides a good statistical average forthe moveout near the source. Common reflection point 102 consists oftraces having six different source-to-detector distances which are largein comparison with source-todetector distances for common reflectionpoint 101. The source-to-detector distances for common reflection point102 are 18 detector intervals, 19 detector intervals, etc., to 23detector intervals. This provides a good statistical average of traceshaving long sourceto-detector distances. A comparison of the traces forcommon reflection points 101 and 102 will disclose errors in the normalmoveout as applied by conventional techniques. Similar conditions applyfor common reflection points 103 and 104, 105 and 106, 107' and 108.Hence, there is a series of comparisons available along the traverse toshow where moveout errors exist. Common reflection points 101 and 102are consecutive common reflection points. A tie at the same commonreflection point can be obtained by recording 25 channels instead of 24channels. The detectors on the added channel would be detector 19 forthe initial source, detector 20 for the next source, etc., to detector24.

FIGS. 10 and 1 1 illustrate an alternative arrangement which employs thesame number of detector stations as the number of recording channels. Noswitching means is employed. In this case illustrated, there are 24detector groups and the distance from the source to the closest detectoris equal to the spacing between detectors. Either of these can bemodified since they are illustrative rather than restrictive. Thetraverse is started with the source at location A and 24 detectors, or24 detector arrays, extending to the right of location A as illustratedin FIG. 10. The source is excited at location A and the outputs ofdetectors 1 through 24 are recorded in the usual manner. The source anddetectors are slowly moved one-half a detector interval to the left.When the source reaches location B, the source is excited and the outputof all 24 detectors, or detector arrays, are recorded. The process isrepeated for location C., etc., through location F. The assemblyconsisting of source and detectors is then moved to the left as fast aspractical past locations G, H, etc., to location K and the source is notexcited at any of these 18 locations. The assembly is slowed down to areasonable speed for recording. The source is excited at location L andthe data from all 24 detectors is recorded. The assembly continues tothe left at a slow constant speed, the source is excited at locations Mthrough Q, and a recording of each source excitation is made from all 24detectors. The speed of the assembly is then increased past locations R,S, etc., to location T. The speed is decreased to the slow constantspeed for recording from locations U through V. The process is repeatedalong the traverse.

FIG. 1 1 is a detail of the initial portion of the process. The initialposition is shown at the top of FIG. 11 with the source at location Aand 24 detectors to the right. After the intial recording, the assemblyis moved slowly to the left as incidacted by the Tow arrow, but the nextsource location B is indicated below the original position for clarity.A slow constant speed is used for recording through location F. Thereflection point for the source at location A and detector 1, is at themidpoint between source A and detector 1 and is indicated by the smallcircle A1. Similar reflection points to the other detectors are to theright, such as, A12 from detector 12 and A24 from detector 24. Thereflection points from the source at location B are shown slightly lowerand to the left. The process continues through the source at location Findicated and the reflection points are F1 to F24. Data from a commonreflection point W is indicated at A12, B13, etc., to F17. There are sixof these paths, hence, six-fold coverage. Reflection points for sourcesat locations L through Q is indicated in the block from L1 to L24 anddown to O1 to Q24. Common reflection point Y has two reflection pointsfrom E and F and four points from L through 0, namely E1, F2, L21, M22,N23 and 024. It can be observed that each common reflection point hassix different reflection paths. The number of detectors could beincreased to obtain an increase in multiplicity of the coverage or todecrease number of locations for sources or similar modifications. Ifthe number of locations for sources is increased in the consecutiveseries of sources, such as A through F, the multiplicity of coveragewould be increased but number of locations skipped would be decreased.Each detector is indicated as a detector but in practice usually anarray of detectors connected together will be used in place of a singledetector. The sources with the effective center of the array at thelocation indicated for the source location. As many possible embodimentsmay be made of this invention without departing from the spirit or scopethereof, it is understood that all matters hereinabove set forth orshown in the accompanying drawings are to be interpreted as illustrativeand not in a limiting sense.

We claim: 1. A method of marine seismic exploration wherein a seismicspread having a plurality of seismic detectors evenly spaced along thelength thereof is towed through a body of water by a vessel, saidseismic spread having leading and trailing extremities, seismic datarecording means being provided and including a lesser number ofrecording channels than the number of said seismic detectors, saidmethod comprising:

towing said spread through the body of water at an optimum constantspeed for efficient data acquisition; generating successive seismicwaves at a shotpoint fixedly located with respect to said seismic spreadas said spread is moved through said body of water, said seismic wavesbeing generated at intervals equal to selected integral multiples of onehalf of the spacing of said seismic detectors; switching said recordingchannels at detector intervals corresponding to said selected integralmultiples along said seismic spread after each seismic wave generationto a different selected group of said detectors; continuing said seismicwave generation and switching of said recording channels during a firstpredetermined length of movement of said seismic spread; discontinuinggeneration of seismic waves and switching of said recording channelsduring a second predetermined length of movement of said seismic spread;and towing said seismic spread through said body of water at a speed inexcess of said optimum constant speed during said second predeterminedlength of movement of said seismic spread. 2. A method according toclaim 1, wherein: said first predetermined length of movement beingsubstantially equal to one-half of the length of said seismic spread. 3.A method according to claim 1, wherein: said second predetermined lengthof movement being substantially equal to one-half of the length of saidseismic spread. 4. A method according to claim 1, wherein: said firstand second predetermined lengths of movement each being substantiallyequal to one-half of the length of said seismic spread. 5. A methodaccording to claim 1 wherein: the number of detectors in said selectedgroup of said detectors being equal to the number of recording channels.

6. A method of marine seismic exploration wherein a seismic spreadhaving a plurality of seismic detectors evenly spaced along the lengththereof is towed through a body of water by a vessel, said seismicspread having leading and trailing extremities, seismic data recordingmeans being provided and including a lesser number of recording channelsthan the number of said seismic detectors, said method comprising:

towing said spread through the body of water at an optimum constantspeed for efficient data acquisition; initiating data acquisition withsaid recording channels connected to the leading ones of said number ofseismic detectors in said seismic spread; generating successive seismicwaves at a shotpoint fixedly located with respect to said seismic spreadas said spread is moved through said body of water; switching saidrecording channels at detector intervals corresponding to said shotpointspacing toward the trailing extremity of said seismic spread after eachseismic wave generation; continuing said seismic wave generation andswitching of said recording channels during a first predetermined lengthof movement of said seismic spread; discontinuing generation of seismicwaves and switching of said recording channels during a secondpredetermined length of movement of said seismic spread; and towing saidseismic spread through said body of water at a speed in excess of saidoptimum constant speed during said second predetermined length ofmovement of said seismic spread. 7. A method according to claim 6,wherein: said seismic waves are generated upon movement of said seismicspread a distance substantially equal to one-half of the distancebetween said detectors. 8. A method according to claim 7, wherein: saidswitching of said recording channels comprises switching each of saidrecording channels one detector interval toward the trailing extremityof said seismic spread after each seismic wave generation. 9. A methodof marine seismic exploration wherein a seismic spread having aplurality of seismic detector means evenly spaced along the lengththereof is towed through a body of water by a vessel, said seismicspread having leading and trailing extremities, seismic data recordingmeans being provided and including a lesser number of recording channelsthan the number of said seismic detectors, said method comprising:

towing said seismic spread through the body of water at an optimumconstant speed for efficient data acquisition; generating successiveseismic waves at a shotpoint located at a particular interval ahead ofthe first of said seismic detector means as said seismic spread is movedthrough said body of water; initiating data acquisition with saidrecording channels connected to the leading ones of said number ofseismic detector means in said seismic spread; switching each of saidrecording channels one detector interval toward the trailing extremityof said seismic spread after each seismic wave generation; continuingsaid seismic wave generation and said switching of said recordingchannels until all of said seismic detectors have been selectivelyactivated;

discontinuing generation of seismic waves and switching of saidrecording channels during a predetermined length of movement of saidseismic spread through the water; and

towing said seismic spread through said body of water at a speed inexcess of said optimum constant speed during said predetermined lengthof movement of said seismic spread.

10. A method according to claim 9 wherein:

said predetermined length of movement of said seismic spread equalssubstantially one-half of the length of said seismic spread.

11. A method according to claim 9 wherein:

said shotpoint is generated at a distance ahead of the first of saidgroup of seismic detectors equal to the average distance between saiddetectors of said seismic spread.

12. A method according to claim 9 wherein:

said shotpoint is generated at a distance ahead of the first of saidgroup of seismic detectors equal to the average distance between saiddetectors of said seismic spread; and

said predetermined length of movement of said seismic spread equalssubstantially one-half of the length of said seismic spread. 13. Amethod of seismic exploration wherein a seismic spread having aplurality of seismic detector means evenly spaced along the lengththereof is moved in one direction, said seismic spread having leadingand trailing extremities, said detectors being capable of recordingreflected seismic data at any time, said method comprising:

moving said seismic spread in said one direction; initiating saidseismic data acquisition with a contiguous group of said detector meansbeing selected to detect and transmit seismic information for recording;

switching said group of selected detector means an equivalent detectorinterval in a direction opposing said one direction subsequent to eachseismic wave generation.

14. A method of seismic exploration wherein a seismic spread having aplurality of seismic detector means evenly spaced along the lengththereof is moved in one direction, said seismic spread having leadingand trailing extremities, said detectors being capable of recordingreflected seismic data at any time, said method comprising:

moving said seismic spread in said one direction;

initiating said seismic data acquisition with a contiguous group of saiddetector means being selected to detect and transmit seismic informationfor ,recording said contiguous group of said detector means beginningwith the first of said detector means;

generating successive seismic waves at source positions being fixedlylocated with respect to said seismic spread as said spread is moved; and

switching said group of selected detector means in a direction opposingsaid one direction subsequent toeach seismic wave generation saidswitching being accomplished by movement of said contiguous group ofsaid active detector means a detector interval corresponding tothe-detector interval of said source positions in a direction opposingmovement of said spread subsequent to each seismic wave generation.

15. A method according to claim 12, wherein:

all of said detector means are active to receive and transmit seismicinformation;

selectively collecting data from a specific contiguous group of saiddetector means; and

said switching of said selected detector means being accomplished byswitching said data collection one detector interval subsequent to eachseismic wave generation.

16. A method of marine seismic exploration for achieving continuousmultiple fold coverage of a subsurface environment wherein a seismicspread having a seismic source and a plurality of seismic detectorsevenly spaced along the length thereof is towed along a traverse havinguniformly spaced locations and a contiguous group of recording channelsis employed to receive signals from selected contiguous groups of saidseismic detectors, said method comprising:

a. towing the seismic spread through the body of water at an optimumconstant speed for efficient data acquisition;

b. successively exciting the seismic source at a first predeterminednumber of consecutive uniformly spaced locations and recording theseismic signals received by the plurality of seismic detectors;

c. switching said recording channels one detector interval along saidseismic spread after each seismic excitation;

d. increasing the speed of the seismic spread through the water withoutrecording seismic signals from a second predetermined number ofconsecutive locations at uniformly spaced locations;

e. reducing the towing speed of the seismic spread to the optimumconstant speed; and

f. successively exciting the seismic source at the first predeterminednumber of consecutive locations at uniformly spaced locations andrecording the seismic signals received by the plurality of seismicdetectors.

17. The method of claim 16 wherein steps (d), (e) and (f) are repeatedalong the traverse.

18. A method of seismic exploration for achieving continuous multiplefold coverage of a subsurface environment wherein a seismic spreadhaving a seismic source and a plurality of seismic detectors evenlyspaced along the length thereof is located along a traverse havinguniformly spaced locations and a contiguous group of recording channelsis employed to receive signals from selected contiguous groups of saidseismic detectors, said method comprising:

a. locating the seismic spread along the traverse with the sourcelocated in relation to one of the uniformly spaced locations;

b. exciting the source and recording the signals received from aselected consecutive series of the plurality of seismic detectors;

c. relocating the seismic spread along the traverse with the seismicsource located in the same relation to the next of the uniformly spacedlocations along the traverse;

d. selecting a different consecutive series of said seismic detectorsfollowing each excitation of said source;

e. repeating step (b) exciting the source and recording the signalsreceived from said different consecutive series of said seismicdetectors;

f. repeating steps (c), (d) and (e) forming a first predetermined numberof recordings from consecutive uniformly spaced locations;

g. skipping a second predetermined number of consecutive uniformlyspaced locations without exciting the source at the locations skipped;and

h. successively repeating step (f) then step (g) along the traverse.

1. A method of marine seismic exploration wherein a seismic spreadhaving a plurality of seismic detectors evenly spaced along the lengththereof is towed through a body of water by a vessel, said seismicspread having leading and trailing extremities, seismic data recordingmeans being provided and including a lesser number of recording channelsthan the number of said seismic detectors, said method comprising:towing said spread through the body of water at an optimum constantspeed for efficient data acquisition; generating successive seismicwaves at a shotpoint fixedly located with respect to said seismic spreadas said spread is moved through said body of water, said seismic wavesbeing generated at intervals equal to selected integral multiples of onehalf of the spacing of said seismic detectors; switching said recordingchannels at detector intervals corresponding to said selected integralmultiples along said seismic spread after each seismic wave generationto a different selected group of said detectors; continuing said seismicwave generation and switching of said recording channels during a firstpredetermined length of movement of said seismic spread; discontinuinggeneration of seismic waves and switching of said recording channelsduring a second predetermined length of movement of said seismic spread;and towing said seismic spread through said body of water at a speed inexcess of said optimum constant speed during said second predeterminedlength of movement of said seismic spread.
 2. A method according toclaim 1, wherein: said first predetermined length of movement beingsubstantially equal to one-half of the length of said seismic spread. 3.A method according to claim 1, wherein: said second predetermined lengthof movement being substantially equal to one-half of the length of saidseismic spread.
 4. A method according to claim 1, wherein: said firstand second predetermined lengths of movement each being substantiallyequal to one-half of the length of said seismic spread.
 5. A methodaccording to claim 1 wherein: the number of detectors in said selectedgroup of said detectors being equal to the number of recording channels.6. A method of marine seismic exploration wherein a seismic spreadhaving a plurality of seismic detectors evenly spaced along the lengththereof is towed through a body of water by a vessel, said seismicspread having leading and trailing extremities, seismic data recordingmeans being provided and including a lesser number of recording channelsthan the number of said seismic detectors, said method comprising:towing said spread through the body of water at an optimum constantspeed for efficient data acquisition; initiating data acquisition withsaid recording channels connected to the leading ones of said number ofseismic detectors in said seismic spread; generating successive seismicwaves at a shotpoint fixedly located with respect to said seismic spreadas said spread is moved through said body of water; switching saidrecording channels at detector intervals corresponding to said shotpointspacing toward the trailing extremity of said seismic spread after eachseismic wave generation; continuing said seismic wave generation andswitching of said recording channels during a first predetermined lengthof movement of said seismic spread; discontinuing generation of seismicwaves and switching of said recording channels during a secondpredetermined length of movement of said seismic spread; and towing saidseismic spread through said body of water at a speed in excess of saidoptimum constant speed during said second predetermined length ofmovement of said seismic spread.
 7. A method according to claim 6,wherein: said seismic waves are generated upon movement of said seismicspread a distance substantially equal to one-half of the distancebetween said detectors.
 8. A method according to claim 7, wherein: saidswitching of said recording channels comprises switching each of saidrecording channels one detector interval toward the trailing extremityof said seismic spread after each seismic wave generation.
 9. A methodof marine seismic exploration wherein a seismic spread having aplurality of seismic detector means evenly spaced along the lengththereof is towed through a body of water by a vessel, said seismicspread having leading and trailing extremities, seismic data recordingmeans being provided and including a lesser number of recording channelsthan the number of said seismic detectors, said method comprising:towing said seismic spread through the body of water at an optimumconstant speed for efficient data acquisition; generating successiveseismic waves at a shotpoint located at a particular interval ahead ofthe first of said seismic detector means as said seismic spread is movedthrough said body of water; initiating data acquisition with saidrecording channels connected to the leading ones of said number ofseismic detector means in said seismic spread; switching each of saidrecording channels one detector interval toward the trailing extremityof said seismic spread after each seismic wave generation; continuingsaid seismic wave generation and said switching of said recordingchannels until all of said seismic detectors have been selectivelyactivated; discontinuing generation of seismic waves and switching ofsaid recording channels during a predetermined length of movement ofsaid seismic spread through the water; and towing said seismic spreadthrough said body of water at a speed in excess of said optimum constantspeed during said predetermined length of movement of said seismicspread.
 10. A method according to claim 9 wherein: said predeterminedlength of movement of said seismic spread equals substantially one-halfof the length of said seismic spread.
 11. A method according to claim 9wherein: said shotpoint is generated at a distance ahead of the first ofsaid group of seismic detectors equal to the average distance betweensaid detectors of said seismic spread.
 12. A method according to claim 9wherein: said shotpoint is generated at a distance ahead of the first ofsaid group of seismic detectors equal to the average distance betweensaid detectors of said seismic spread; and said predetermined length ofmovement of said seismic spread equals substantially one-half of thelength of said seismic spread.
 13. A method of seismic explorationwherein a seismic spread having a plurality of seismic detector meansevenly spaced along the length thereof is moved in one direction, saidseismic spread having leading and trailing extremities, said detectorsbeing capable of recording reflected seismic data at any time, saidmethod comprising: moving said seismic spread in said one direction;initiating said seismic data acquisition with a contiguous group of saiddetector means being selected to detect and transmit seismic informationfor recording; switching said group of selected detector means anequivalent detector interval in a direction opposing said one directionsubsequent to each seismic wave generation.
 14. A method of seismicexploration wherein a seismic spread having a plurality of seismicdetector means evenly spaced along the length thereof is moved in onedirection, said seismic spread having leading and trailing extremities,said detectors being capable of recording reflected seismic data at anytime, said method comprising: moving said seismIc spread in said onedirection; initiating said seismic data acquisition with a contiguousgroup of said detector means being selected to detect and transmitseismic information for recording said contiguous group of said detectormeans beginning with the first of said detector means; generatingsuccessive seismic waves at source positions being fixedly located withrespect to said seismic spread as said spread is moved; and switchingsaid group of selected detector means in a direction opposing said onedirection subsequent to each seismic wave generation said switchingbeing accomplished by movement of said contiguous group of said activedetector means a detector interval corresponding to the detectorinterval of said source positions in a direction opposing movement ofsaid spread subsequent to each seismic wave generation.
 15. A methodaccording to claim 12, wherein: all of said detector means are active toreceive and transmit seismic information; selectively collecting datafrom a specific contiguous group of said detector means; and saidswitching of said selected detector means being accomplished byswitching said data collection one detector interval subsequent to eachseismic wave generation.
 16. A method of marine seismic exploration forachieving continuous multiple fold coverage of a subsurface environmentwherein a seismic spread having a seismic source and a plurality ofseismic detectors evenly spaced along the length thereof is towed alonga traverse having uniformly spaced locations and a contiguous group ofrecording channels is employed to receive signals from selectedcontiguous groups of said seismic detectors, said method comprising: a.towing the seismic spread through the body of water at an optimumconstant speed for efficient data acquisition; b. successively excitingthe seismic source at a first predetermined number of consecutiveuniformly spaced locations and recording the seismic signals received bythe plurality of seismic detectors; c. switching said recording channelsone detector interval along said seismic spread after each seismicexcitation; d. increasing the speed of the seismic spread through thewater without recording seismic signals from a second predeterminednumber of consecutive locations at uniformly spaced locations; e.reducing the towing speed of the seismic spread to the optimum constantspeed; and f. successively exciting the seismic source at the firstpredetermined number of consecutive locations at uniformly spacedlocations and recording the seismic signals received by the plurality ofseismic detectors.
 17. The method of claim 16 wherein steps (d), (e) and(f) are repeated along the traverse.
 18. A method of seismic explorationfor achieving continuous multiple fold coverage of a subsurfaceenvironment wherein a seismic spread having a seismic source and aplurality of seismic detectors evenly spaced along the length thereof islocated along a traverse having uniformly spaced locations and acontiguous group of recording channels is employed to receive signalsfrom selected contiguous groups of said seismic detectors, said methodcomprising: a. locating the seismic spread along the traverse with thesource located in relation to one of the uniformly spaced locations; b.exciting the source and recording the signals received from a selectedconsecutive series of the plurality of seismic detectors; c. relocatingthe seismic spread along the traverse with the seismic source located inthe same relation to the next of the uniformly spaced locations alongthe traverse; d. selecting a different consecutive series of saidseismic detectors following each excitation of said source; e. repeatingstep (b) exciting the source and recording the signals received fromsaid different consecutive series of said seismic detectors; f.repeating steps (c), (d) and (e) forming a first predetermined number ofrecordings from consecutive uniformly spaced locations; g. skipping asecond predetermined number of consecutive uniformly spaced locationswithout exciting the source at the locations skipped; and h.successively repeating step (f) then step (g) along the traverse.